Piezoelectric materials have been used in transducers to convert mechanical strain to electrical charge for energy scavenging applications. Piezoelectric energy harvesting in a so called bimorph configuration is the most popular approach which scavenges mechanical energy for generating electrical energy. Piezoelectric energy harvesters are usually vibrating cantilevers covered with a layer of piezoelectric material. The piezoelectric material converts mechanical strain, for example from environmental vibrations, into a charge that can power an electrical device. Alternatively such devices may operate in reverse for converting electrical energy to mechanical energy or work, for example in a motor or actuator application. Typically, the length of the cantilever in such arrangements is covered with piezoelectric material in a single continuous strip.
Energy harvesting systems based on mechanical-to-electrical conversion technologies have attracted considerable research interest in recent years, particularly for powering wireless sensors. Research has involved energy harvesting methodology based on transduction techniques including piezoelectric, electromagnetic and electrostatic. Of these, piezoelectric transduction is considered the most promising technology in this field and has attracted significant research attention as it generally has higher electromechanical coupling efficiency and requires no external voltage sources compared with other technologies. Piezoelectric transduction is particularly attractive in application areas such as Micro-Electro-Mechanical-Systems (MEMS) and Wireless Sensor Networks (WSNs). Wireless sensor networks have the potential to provide significant advantages compared with existing wired methodologies in various fields of application including: environmental, health, security and military applications due to their flexibility, ease of implementation and operational capability in harsh operational environments. Currently, most WSNs use a battery, rechargeable or otherwise, for power which can limit their application due to high cost, bulk, size and short operational life. Development of energy sources from environments (for example, to convert mechanical vibration to electricity) to power WSNs or microsystems has therefore attracted increasing interest in recent years.
A major factor preventing piezoelectric power harvesting devices from broad practical application is the small amount of power that is generated by known piezoelectric materials.
Recent research activity has been directed towards improving the energy efficiency and life of piezoelectric energy harvesting devices. This can be achieved by: increasing the bandwidth; tuning the resonance frequency of the cantilever; selecting appropriate material(s) such as those with higher coupling efficiency, and improving AC-DC conversion circuitry. The structure of the device has been recognized as a key factor affecting efficiency. Currently, the cantilever beam configuration is the most popular structure for such devices. A cantilever configuration with piezoelectric material attached to the top or bottom of substrate is currently considered the optimum structure for energy harvesting in terms of its simplicity and good coupling efficiency. Conventional cantilever energy harvesting configurations are based on a bulk piezoelectric plate or layer. However, known piezoelectric cantilever beam based harvesting devices predominately work in bending motions/modes of vibration. This is because in the pure torsional modes, the generated charges on the surface of the structure are categorised into two groups, positive and negative charge area. These areas have the same amount of charge with opposite signs due to the symmetrical strain of the cantilevered beam structure in torsion. Thus the total electrical output from the surface of devices of known configurations in torsion will be zero as both positive and negative areas cancel each other out, providing zero net charge.
There is a requirement therefore for an improved piezoelectric energy harvesting device, and/or power consumption when in a reverse, motor/actuator, mode of operation.